The invention relates to polylactic acid-based blends.
Succinic acid and diols can form biodegradable aliphatic polyesters and copolyesters through coupling and polycondensation reactions. The main unit structure resulting from these reactions is: ##STR1##
Examples of biodegradable aliphatic polyesters and copolyesters having the unit structure shown above are polybutylene succinate (PBSU), where m is 4 and n is 2, polyethylene succinate (PESU), where m is 2 and n is 2, a random copolymer of polybutylene succinate adipate (PBSU-AD) where m is 4 and n is 2 or 4, and polyethylene succinate adipate (PESU-AD) where m is 2 and n is 2 or 4.
These polyesters and copolyesters have interesting properties including biodegradability, melt processability, and thermal and chemical resistance. One of these, BIONOLLE.RTM., a commercially available aliphatic succinate-adipate polyester, has excellent physical properties. For example, the thermal resistance of BIONOLLE is equivalent to that of polyethylene, but the yield strength is higher than polyethylene. The stiffness of BIONOLLE is between high density and low density polyethylene (LDPE). Particularly for BIONOLLE #3000, its impact strength is equivalent to that of LDPE, while its elongation at break is higher than that of LDPE.
Polylactic acid can be made from lactic acid (lactate). Lactic acid is a natural molecule that is widely employed in foods as a preservative and a flavoring agent. It is the main building block in the chemical synthesis of the polylactide family of polymers. Although it can be synthesized chemically, lactic acid is procured principally by microbial fermentation of sugars such as glucose or hexose. These sugar feed stocks can be derived from potato skins, corn, and dairy wastes. The lactic acid monomers produced by fermentation are then used to prepare polylactide polymers.
Lactic acid exists essentially in two stereoisomeric forms, which give rise to several morphologically distinct polymers: D-polylactic acid, L-polylactic acid, D,L-polylactic acid, meso-polylactic acids and any combinations of thereof. D-polylactic acid and L-polylactic acid are stereoregular polymers. D,L-polylactic acid is a racemic polymer obtained from a mixture of D- and L-lactic acid, and meso-polylactic acid can be obtained from D,L-lactide. The polymers obtained from the optically active D and L monomers are semicrystalline materials, but the optically inactive D,L-polylactic acid is amorphous.
Lactic acid has a hydroxyl group as well as a carboxylic group, and hence can be easily converted into a polyester. These polyesters have some potential advantages when compared to other biodegradable polymers such as polyhyroxybutyrate and polycaprolactone, as to their strength, thermoplastic behavior, biocompatibility, and availability from renewable sources, and have been classified as "water sensitive," because they degrade slowly compared with "water soluble" or "water swollen" polymers. However, while polylactic acid is a biodegradable polymer with generally good processability, it is brittle, and the brittleness increases with time due to physical aging, i.e., densification of the polymer at a molecular level.